Remove new_issue_url from external issue trackers
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## Summary
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Please briefly describe what part of the code base needs to be refactored.
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Follow-up from https://gitlab.com/gitlab-org/gitlab/-/merge_requests/58899#note_549891689:
> it looks like `new_issues_url` isn't used anymore, but I'm not completely sure.
>
> - The line in the docs was added in https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/16353, unfortunately without any context.
> - I think we stopped using it with https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/12130, previously we'd redirect `/issues/new` if an external issue tracker was configured.
>
> After enabling the Redmine integration I still get the GitLab issue form everywhere, so at least that part seems to be gone. Also found this feature request to bring it back, which I guess confirms that :wink: https://gitlab.com/gitlab-org/gitlab-foss/-/issues/47736
>
> The same probably applies to other external issue trackers with a `new_issue_url`:
>
> - https://docs.gitlab.com/ee/user/project/integrations/bugzilla.html
> - https://docs.gitlab.com/ee/user/project/integrations/ewm.html
> - `app/models/project_services/custom_issue_tracker_service.rb` (no docs)
>
> The one exception seems to be Jira, which still links to `new_issue_url` in the issue list (but it's automatically generated from the Jira base URL).
>
> We can probably go ahead and remove the field completely, I'll open a separate issue. For this MR I'd say let's just keep it as it is currently :sweat_smile:
## Improvements
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See also https://about.gitlab.com/handbook/values/index.html#say-why-not-just-what
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Remove `new_issue_url` from:
- `app/models/project_services/issue_tracker_service.rb`
- `app/models/project_services/issue_tracker_data.rb`
- `app/models/project_services/bugzilla_service.rb`
- `app/models/project_services/custom_issue_tracker_service.rb`
- `app/models/project_services/ewm_service.rb`
- `app/models/project_services/redmine_service.rb`
Also update the related docs, and check that we're not exposing the field in the API.
Docs pages to update:
- `doc/api/services.md`
- `### Create/Edit Bugzilla service`
- `### Create/Edit Custom Issue Tracker service`
- `### Create/Edit Redmine service`
- `doc/user/project/integrations/bugzilla.md`
- `doc/user/project/integrations/custom_issue_tracker.md`
- `doc/user/project/integrations/ewm.md`
- `doc/user/project/integrations/redmine.md`
## Risks
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Please list features that can break because of this refactoring and how you intend to solve that.
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- We haven't fully verified that the URL isn't used anymore, but so far it seems likely.
- `JiraService` also has a `new_issue_url` method, but it's hard-coded so it shouldn't be affected by this change.
issue